The present invention relates to the use of underprinting fluid in inkjet printing, and, more specifically, to chemical compositions of underprinting fluid, which combined with inkjet ink, achieve improvements in both color and stability of inkjet printed images.
The use of inkjet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today""s inkjet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving inkjet print quality, while further lowering cost to the consumer.
An inkjet image is formed when a precise pattern of dots is ejected from a drop-generating device known as a xe2x80x9cprintheadxe2x80x9d onto a printing medium. The typical inkjet printhead has an array of precisely formed nozzles located on a nozzle plate and attached to an inkjet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each chamber has a thin-film resistor, known as a xe2x80x9cfiring resistor,xe2x80x9d located opposite the nozzle so ink can collect between the firing resistor and the nozzle. In particular, each resistor element, which is typically a pad of a resistive material, measures about 35 xcexcmxc3x9735 xcexcm. The printhead is held and protected by an outer packaging referred to as a print cartridge, i.e., inkjet pen.
Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, transparent film or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, thereby forming alphanumeric and other characters on the print medium.
Inks normally used in inkjet recording are commonly composed of water-soluble organic solvents (humectants, etc.), surfactants, and colorants in a predominantly aqueous fluid. When a recording is made on xe2x80x9cplain paperxe2x80x9d, the deposited colorants retain some mobility, which can be manifest in bleed, poor edge acuity, feathering, and inferior optical density/chroma (due to penetration of the paper). These features adversely impact text and image quality.
The evaporation rate of water-based inks is low and they dry primarily by the adsorption into paper. The adsorption time of an ink is dependent on many factors, such as the ink drop size, paper mesh size, ink viscosity, and the ink spreading coefficient over the paper (that is, the surface tension of the paper minus the surface tension of inks and the interfacial tension of the paper-ink interface). There is a considerable interest in reducing the ink drying time, in particular, for high throughput printers. However, shortening the drying time is normally attained at the expense of the image quality. Thus, adding surfactants to inks can significantly reduce the penetration time. This however decreases the optical density/chroma and the edge acuity of the image, as well as ultimately leading to strikethrough problems, because of the penetration of the colorants in normal and lateral directions of the paper.
To address these problems, methods have been developed in which a xe2x80x9cfixerxe2x80x9d solution, with components to reduce colorant mobility, is deposited on the paper prior to depositing the ink. This xe2x80x9cunder-printingxe2x80x9d of imaging ink uses compositions containing acids, salts, and organic counter ions and polyelectrolytes. Underprinting is defined as applying a transparent liquid on paper just before applying inks. The idea is to apply xe2x80x9ca separate reactive componentxe2x80x9d before the inks in order to improve waterfastness. The xe2x80x9creactive componentxe2x80x9d reacts with a reactant present in the inks, producing a polymer that binds the colorant and makes it waterfast. Bifunctional acylchlorides and bifunctional amines, for example, are possible reactive pairs. However, acylchlorides are very reactive and require a nonaqueous ink vehicle. Another example of a reactive pair includes carboxymethylcellulose in the presence of aluminum salt which together form a gel. This reaction is more benign than the first, but has the drawback that carboxymethylcellulose solutions have a high viscosity even without the polyvalent ions present. This makes it difficult to use in an inkjet.
In other disclosures, the underprinting idea has been further developed. For example, Using a cationic xe2x80x9cliquid compositionxe2x80x9d such as a polyallylamine for underprinting of anionic dyes has been used to achieve a fixer, which reacts broadly with dye based systems as a whole.
A broad class of underprinting liquids for polymer dispersed pigments has also been disclosed, including polymer latexes, silica, alumina and titanium oxide particles, polymer resins, buffer solutions, and inorganic salts. All these underprinting liquids destabilize the pigment dispersions by various mechanisms. As a result, the pigment substantially precipitates at the surface of the paper, while the vehicle is quickly adsorbed.
In another system has been described that uses water-soluble dyes and an underprinting fixer, the fixer contains ligand-complexed metal ions (metal ion with an associated ligand). The ink used in this system contains an anionic component, which may be the dye itself. The ink may also contain a polymeric viscosity modifier. The specific polymeric viscosity modifier can be an ethylene oxide adduct of acetylene glycolxe2x80x94a neutral polymer which unlike the present invention does not interact strongly with the fixer.
Another system disclosed uses dispersed pigment colorants and requires a glycol ether cosolvent in the ink. This is unlike the system of the present invention, which uses water soluble pigment colorants with little cosolvent restriction.
The present invention relates to an inkjet ink composition comprising:
an aqueous solvent and at least one pigment,
wherein the at least one pigment comprises macromolecular chromophores having a zeta potential of from 100 to 900 millivolts;
and wherein, when the ink composition and an underprinting fixer fluid are mixed together to form a mixture, a portion of the at least one pigment in the ink composition precipitates out of the mixture;
and wherein the fixer fluid comprises at least one cationic component.
The present invention also relates to an underprinting fixer fluid comprising:
at least one cationic component,
wherein, when the fixer fluid and an inkjet ink comprising at least one pigment and an aqueous solvent are mixed together to form a mixture, a portion of the at least one pigment precipitates out of the mixture, and wherein the at least one pigment comprises macromolecular chromophores having a zeta potential of from 100 to 900 millivolts.
Furthermore, the present invention relates to a method of inkjet printing, the method comprising the steps of:
a) ejecting at a pixel location on a medium an ink composition comprising at least one pigment; and
b) immediately before and/or immediately after ejecting the ink composition, ejecting at the pixel location on the medium an underprinting fixer fluid comprising at least one cationic component;
wherein, when the fixer fluid and an inkjet ink comprising at least one pigment are mixed together to form a mixture, a portion of the at least one pigment precipitates out of the mixture;
and wherein the at least one pigment comprises macromolecular chromophores having a zeta potential of from 100 to 900 millivolts.
Also, the present invention relates to an inkjet printing apparatus comprising:
at least one printhead portion, the printhead portion having at least two ejector portions; and
at least one reservoir portion having at least two reservoir chambers, each reservoir chamber providing fluid to one of the at least two ejector portions, and at least one of the reservoir chambers including an underprinting fixer fluid comprising at least one cationic component;
and at least one of the reservoir chambers including an ink composition comprising an aqueous solvent and at least one pigment;
wherein when the underprinting fixer fluid and the ink composition are mixed together to form a mixture, a portion of the at least one pigment in the ink composition precipitates out of the mixture;
and wherein the at least one pigment comprises macromolecular chromophores having a zeta potential of from 100 to 900 millivolts. In addition, the present invention relates to a fluid delivery apparatus for providing fluid to an inkjet printing system, the fluid delivery apparatus comprising: a reservoir portion having at least two reservoir chambers, each reservoir chamber providing fluid to one of the at least two ejector portions, and at least one of the reservoir chambers including an underprinting fixer fluid comprising at least one cationic component; and at least one of the reservoir chambers including an ink composition comprising an aqueous solvent and at least one anionic pigment;
wherein when the underprinting fixer fluid and the ink composition are mixed together to form a mixture, a portion of the at least one anionic pigment in the ink composition precipitates out of the mixture;
and wherein the at least one pigment comprises macromolecular chromophores having a zeta potential of from 100 to 900 millivolts.